1. Field of the Invention
The invention relates to a process for the manufacture of mixed esters from lactic acid, citric acid and partial glycerides of fatty acids whose free carboxyl groups are neutralized with alkali to give the mixed ester an acid number of 10 to 200. The term fatty acids means an alkyl carboxylic acid with 8 to 18 carbon atoms, which may be saturated or unsaturated.* FNT * The term partial glycerides means mono-and/or di-esters of glycerine.
2. Description of the Prior Art
The use of esters of hydroxycarboxylic acids and partial glycerides of fatty acids for solving emulsifying and wetting problems in the fields of pharmacy, cosmetics, foods, and animal foods, is well known. At the same time, the use of hydroxycarboxylic acids, which are known to be absolutely safe physiologically, is both partly desirable and partly necessary. Such hydroxycarboxylic acids, are, for example, lactic acid and citric acid. The manufacture of such esters with a high degress of hydrophilicity however presents difficulties.
German Auslegeschrift No. 12 78 423 discloses the direct reaction of partial glycerides of fatty acids with citric acid. This process however has serious disadvantages and its applicability is therefore only limited. For example, it is possible to react at most only one mole of citric acid with one mole of the monoglyceride of a fatty acid. If the molar proportion of citric acid is increased, for the most part, it does not react and precipitates from the reaction product. If attempts are made to force the reaction by increasing the temperature, the citric acid undergoes uncontrollable decomposition which lowers the hydrophilicity of the reaction product. Because of the by-products formed, the process products no longer are absolutely safe physiologically.
It is also known that citric acid may be converted initially into an internal acid anhydride by reaction with acetic anhydride, the acetyl compound being formed at the same time and that this internal anhydride may be esterified with the partial esters of fatty acids. In this case however, one does not obtain the pure citric acid ester of the partial monoglyceride. Rather products are obtained in which the hydroxyl group of the citric acid is acetylated. When these reactions products are stored or used, acetic acid is eliminated and impairs the odor and taste of products, to which these reaction products have been added.
The object of German Patent No. 24 55 989 is a process for the manufacture of citrates of monoglycerides and/or diglycerides of fatty acids at temperatures of 100.degree. to 140.degree. C. The process is characterized by the fact that the reaction is carried out in the presence of amounts of acetic acid such that a clear solution is formed and is maintained during the reaction.
With this process, it has become possible to prepare citrates of partial glycerides of fatty acids with a higher degree of conversion than in the German Auslegeschrift No. 12 78 423. However, in this case it is also necessary to use acetic acid which insofar as the desired process products are concerned is extraneous.
Frequently it is desirable to increase the hydrophilicity of such esters even further, in order to influence the emulsifying action and to improve not only the solubility of the esters in cold water, but also, the so-called cold emulsifying ability. The partial or complete neutralization of the three carboxyl groups present in the esters with alkali is well known for this purpose. In this way, the pH value of the aqueous dispersion or solution is further increased and the stability of the esters in aqueous solution is improved by repressing hydroysis or saponification reactions.
A direct neutralization of the carboxyl groups of the citrate in the molten state with alkali hydroxide does not go to completion because of the partial saponification which takes place. If alkali carbonate is used, severe foaming is observed, which from a practical standpoint prevents the neutralization.
If the esters containing the three carboxyl groups are melted and mixed with aqueous solutions of alkali hydroxide or carbonate, the partial saponification can still not be avoided. Here also, foaming is quite noticeable. If the solutions are dilute, in order to check the saponification reaction, at relatively low temperatures, and especially below 40.degree. C., the water or other solvent used must be removed again after the neutralization since the products should not be in the form of a solution for use in technical applications. Rather, they should be in solid, finely powdered form and especially in a flowable form. This is possible if a partial glyceride of a higher saturated fatty acid has been used.
Admittedly it is known to react such esters with pulverulent basic alkali salts, so as to retain the pulverulent state of the product. Such a product gives a neutral reaction after the addition of water. In any case, the high electrolyte content of such products interferes in many end use applications.